One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (“ABS”) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
An actuator is used to move the read/write head or transducer. A voice coil motor (VCM) is used to move the actuator. The VCM is a Lorentz type actuator and hence linear in its characteristics. In most disc drives, the coil of the voice coil motor is attached to the actuator assembly. Most rotary actuators have a y-shaped yoke which holds a voice coil located at one end of the actuator. At least one magnet is positioned near the voice coil on the yoke to form a voice coil motor. The VCM typically comprises a loop of copper wire in a magnetic circuit. Application of current to the coil leads to generation of a Lorentz force in the actuator. A bearing cartridge is placed at the pivot point of the actuator so that the actuator pivots about the bearing. Due to the offset between the force axis and the pivot axis, a resultant torque is induced which rotates the VCM. This design has many shortcomings. First, this is a high inertia design which lowers resonant frequencies of the actuator. Higher resonant frequencies are desired in actuators so as to place the resonant frequencies to be outside the normal operating range of the actuator. Also, there is a restriction on the amount of current input to the coil because of the absence of a heat sink to conduct heat away from the coil. The yoke of current voice coil motors is made as light and with as little an amount of material as possible. Such a yoke is just the opposite of a good heat sink which can remove heat from the coil. The most significant shortcoming however is that the torque used to move the rotary actuator does not act through the pivot point, but is a result of off-axis forces. While the off-axis forces result in a torque about the pivot, they also result in a force through the pivot point which moves the actuator assembly slightly to one side or the other. This mode of motion of the actuator is called the primary mode or the actuator translation mode. The amount of motion due to this mode is slight, but the slight motion is significant since the track density on the disc is so high. For example, current designs have track densities as high as 58000 tracks per inch (“TPI”). In essence, the track width is very small and so side motion is significant and results in track misregistration. There is also a constant push to increase areal density which means that even higher track densities are anticipated in the future. As a result, this problem will get worse as the track densities increase.
The actuator translation mode also called the primary mode is also the biggest limiting mechanism to implementing high bandwidth controllers on the VCM axis. The current VCM compounds this problem in two ways. First of all, the current VCM is not a true torque actuator and the off-pivot axis force results in not only a torque τ about the pivot, but also a lateral force Fτ that directly excites the actuator translation mode. This causes increased non-repeatable runout (NRRO). This phenomenon is best illustrated and discussed in FIG. 2 of this application. Secondly, the presence of a high density and low specific strength material such as copper far away from the pivot axis increases both the mass and the polar moment of inertia of the actuator significantly. Without corresponding increases in the stiffness of the bearing cartridge, this increased mass leads to a reduction in the primary translation mode. This in turn limits achievable bandwidth and also leads to error components in the NRRO spectrum.
Another problem related to the inertia of the current VCM is that it has a high moment of inertia which limits the speed of the seeks can be accomplished. Seeks require the actuator to change directions and stop over a track. Higher inertia VCMs take a longer time to change direction and a longer time to settle over a track to assure that the transducer is actually over the desired track from which data will be obtained or to which information representing data will be written.
What is needed is a disc drive which provides for either elimination or a substantial reduction in the translation mode of excitation and which also allows for faster seek times and improved bandwidth capability on read and write operations. In addition, there is a need for a stiffer mechanical structure by eliminating the coil of the voice coil magnet hanging off one end of the actuator assembly. There is also a need for improved heat sink for the coil of the voice coil motor and an efficient use of the flux from the magnets. There is also a need for a voice coil magnet that will not produce a flux pattern that will interfere with magneto resistive heads and which will not interfere with information representing data which is stored on a disc. There is also a need for an actuator assembly that has a lower moment of inertia than current designs which also would allow for faster access times. There is also a need for a true torque motor that will not translate with respect to the pivot.